This invention relates to the fabrication of emitter-coupled logic circuits which are compatible with transistor-transistor logic circuits and more particularly to a compound diffusion process for fabricating both transistor-transistor logic transistors and emitter resistors used with the emitter coupled logic circuit, which compound diffusion eliminates spiking problems associated with emitter resistors.
One of the most important parameters of computer systems is the speed at which the computer can operate.
In view of the growth of sophistication in the computer industry, it has been found that emitter coupled logic (ECL) circuits provide computers with as must as three times the speed of conventional circuits.
It will be appreciated that ECL circuit modules have application where computer speed is of the essence. This occurs in real time tracking and data acquisition applications as well as in the look ahead circuits utilized to enable the computer to quickly check on its operation at various intervals in a computer program. In general, ECL circuits may be utilized to improve the available capacity of any computer.
The speed of a computer is usually indicated by propagation delay such that computers utilizing emitter coupled logic circuits operate in the 150 to 300 megahertz range as compared to saturated logic circuits involving transistor-transistor logic (TTL) which operate in the 50 megahertz range. The increased speed which is available with emitter coupled logic circuits is due to the fact that the transistors utilized do not saturate in either of their two logic states. These transistors are therefore never turned off. In ECL circuits switching is accomplished between one active current level and another active current level. Contrasted with this are the TTL transistors. The reason for the slower speed of TTL circuits lies in part in the fact that the TTL transistors go from one logic state to another by turning OFF and ON, which takes time.
Another reason for the speed of the emitter coupled logic circuits is the physical size of the circuit itself. It will be appreciated that size addes parasitic capacitance which in general slows the speed at which the logic can operate. Thus computers using circuits of smaller size have a decided speed advantage. A subsidiary yet important consideration is that the cost of the chip is proportional to its size. Emitter-coupled logic circuits, because of their speed, must be made using smaller components and therefore can be fabricated with more chips to the wafer. This not only reduces the parasitic capacitance problem but also achieves a much lower chip cost. Further size is important since the length of the circuits in the computer is an absolute limit on the speed of the computers. Speed is limited by the absolute length of the circuit because of the propagation time of an electrical signal through the circuit from an input to an output. Since modern computers are now approaching the absolute theoretical limit to speed, it is important that the active elements in the computer be reduced in size as much as possible. One further consideration of the size reduction is power consumption. In general, emitter-coupled logic circuits draw more power than their associated transistor-transistor logic circuits. However, by changing the resistor configurations in an ECL circuit, power consumption for these circuits is brought into line with that dissipated by equivalent transistor-transistor logic circuits.
Reduction in size of these ECL circuits requires different geometries and designs. Since one of the major factors in the size of an ECL circuit is the size of the resistors utilized, the making of smaller resistors having the requisite resistance values is significant in the reduction of the size of ECL circuits.
A further consideration which indicates the use of ECL circuits is in part the logic system itself which utilizes simpler logic configurations than is possible with transistor-transistor logic circuits. This makes possible the reduction of the number of transistors necessary for a given logic function. However, since the majority of computers already constructed utilize transistor-transistor logic, a transistor-transistor logic compatibility must be built in both at the input to the ECL circuits and at the output of the ECL circuits so as to make the ECL circuits. compatible with the TTL circuits. This is necessary because the ECL logic swings utilize negative voltages while the TTL circuits utilize positive voltage swings. Further the magnitude of the logic swings is different in each case. Those circuits which provide for this compatibility are called "translators" and are built into the ECL chip. The building of TTL components into an ECL chip is not a simple matter and usually requires a large number of extra processing steps.
This invention is therefore directed to a method for fabricating an integrated circuit chip having both ECL and TTL circuits combined with resistors whose circuit areas can be minimized.
In the reduction of size of an ECL circuit chip, it will be appreciated that the low valued resistors utilized as input resistors for ECL transistors range from 50 to 250 ohms. In order to provide these values by conventional diffused resistor techniques, the resistors take up approximately 45 percent of the chip area. It has recently been suggested that emitter resistors be utilized in place of large area resistors to provide the needed resistance values. If emitter resistors are utilized, only 25 percent of the chip area need be utilized for the resistors.
An emitter resistor is one which is formed by diffusing an emitter tub region into a base tub region which base tub region may also be diffused into an epitaxial layer. The resistance is developed solely across the emitter tub region utilizing the base tub region for isolation. The base tub permits the use of 15 ohm/sq. material for the resistors so that the resistor area can be made small. Thus the base isolation tub can be small and no isolating ring is needed. The emitter resistor area can be made small because 15 ohm/sq. material can carry more current per unit area than 100 ohm/sq. material. This permits narrow resistor tubs and increased packing density.
There are, however, serious problems involved in the utilization of emitter resistors in emitter-coupled logic circuits. The primary problem with emitter resistors is referred to herein as "spiking". Spiking refers to a current leakage problem which is thought to be due to lattice damage or imperfections in the base region of the emitter resistor. It is thought that this damage occurs due to high impurity concentrations from the diffused emitter region which extends into the base tub region. The spiking problem is so severe that ECL circuits utilizing emitter resistors have yields of only 14 to 16 percent. This is contrasted with yields of 40 percent when utilizing the subject compound diffusion technique. The current leakage associated with convention emitter resistors is of such a nature that current leakage causes a latch problem in the circuit. This latch problem results in the pulling down of the value of these input resistors from a preset value to a much lower value which throws a given circuit out of its particular logic state; that is the state which it should be in, absent the current leakage problem.
The solution to this spiking problem is the provision of a compound tub diffusion, sometimes called a base diffusion, which all but completely eliminates the spiking problem. The reason that the compound diffusion eliminates the spiking problems is not yet known. It is thought, however, that due to the compound diffusion both a higher impurity concentration is available at the base emitter junction for the emitter resistor and a more uniform surface for the base region is provided. The elimination of spiking may also be due to not only the physical structure which results after the compound diffusion, but also the manner in which the diffusions are made. While it cannot be said that any one step is responsible for the elimination of spiking, the compound diffusions are characterized by a predeposition of a dopant which dopant is in saturated solution at the surface of the substrate to be doped.
The term "compound diffusion" refers to a double diffusion in which the region over the base tub is opened up for two different diffusion cycles in which material of a first impurity concentration is first diffused followed by a diffusion of a material of a second impurity concentration. The resulting impurity concentration in the base isolation region results in a sheet resistivity of 60 ohms per square which sheet resistivity is exactly that which is necessary for the base region of the transistor-transistor logic transistors utilized as the level translators for the ECL circuit. The compound diffusion is accomplished from two separate diffusions which result in surface sheet resistivities of 100 ohms per square and 300 ohms per square when done in two different substrate regions. Because of the difficulty in matching doping profiles and predicting the outcome of compound diffusions, the 60-ohm per square sheet resistivity is not predictable from doing the above two diffusions in the same region of the substrate.
Unexpectedly these two diffusions when done sequentially at the same location result not only in a suitable resistor base region, but also in a region suitable as a base region for TTL transistors. Further, one of these diffusions results in a base region uniquely suited to ECL transistors.
To elaborate, it is a requirement that the TTL transistors have low betas which center around betas of 30. The reason for these low betas is that with saturated logic circuits (TTL) the beta of the transistors must be low in order to reduce device response time. On the other hand, with ECL circuits, the betas must be relatively high and on the order of 150. The ECL circuits require high gain transistors to keep the base current of the transistors as low as possible to conserve on power. The ECL transistors of the subject circuit are typically made by a single diffusion which results in a region having a sheet resistivity of 300 ohms/sq. The TTL transistors are however in need of 60-ohm per square resistivity regions. In the subject process both the emitter resistors and the TTL transistors are made by the compound diffusion process which if done separately in different locations yield regions having resistivities of 100 ohms per square and 300 ohms per square.
As a by-product of the compound diffusion process and because material having a sheet resistivity of 100 ohms per square is made, there is available the 100-ohm per square resistivity for conventional resistors whose values do not need to be as low as 50 to 250 ohms. It will be appreciated that higher valued resistors can be diffused into the chip substrate without too much of a sacrifice in area since their areas are small.
The use of the compound diffusion technique is not obvious over prior art techniques with respect to ECL circuits for at least six reasons. The first of these reasons is that it is not obvious that appropriate base concentrations for TTL devices can be obtained by compound diffusion. The reason that it is not obvious is because compound diffusions rarely have a direct additive effect. This means that it is relatively difficult to predict a final base concentration given two different diffusions. It will be further appreciated that junction profiles and the resulting resistivities cannot easily be calculated theoretically because of the combined impurity profile and the intermediate oxidation steps. The oxide layers resulting from these oxidations in part provide a carrier for the dopants which further complicates prediction of combined impurity concentrations. Secondly, even if this were obvious, it would certainly not be obvious that one could obtain appropriate base concentrations for TTL circuits utilizing one doping concentration which is uniquely appropriate for ECL transistors. Thirdly, it is not obvious that compound diffusion will yield emitter resistors having controllable resistance values, so little is there known about the compound diffusion process. Fourthly, even if it were obvious that the value of emitter resistors could be controlled by compound diffusions, it is not obvious that spiking can be eliminated by any type of compound diffusion process. Fifthly, that one can obtain the elimination of spiking by making materials having TTL and ECL compatible base concentrations is not obvious. Sixthly and finally, by utilizing TTL and ECL compatible base concentrations is not obvious that a total integrated circuit package can be formed with a minimum of processing steps due to the compatibility of the compound diffusion process with the single diffusion processes now utilized in making TTL and ECL devices in discrete areas of the chip. It is the combination of the above effects which enable the same type base region to be formed for TTL transistors and emitter resistors. It is also the above combination which permits the formation of the entire ECL circuit (including translation devices) in two diffusion steps with one diffusion being appropriate for ECL components while the second diffusion enables the formation of the TTL elements, the emitter resistors and standard diffused resistive elements. It should be noted that it is the discovery that spiking can be eliminated in emitter resistors that makes possible the reduction of chip area by as much as 20%.